资源环境科技发展态势分析平台

An objective definition of monsoon gyres in the northwest Pacific was developed in order to construct a gyre climatology. Over a 31- yr period, 53 gyres were identified with a median formation location at 16.5 degrees N, 135 degrees E. More than 80% formed during July-September. More than half of gyres developed during El Nino periods at a median location 1200 km farther to the east-southeast than during La Nina. Cyclonic winds at 850 hPa extended across a diameter of more than 4000 km, with maximum tangential wind near the 1000-km radius. A precipitation maximum extended westward for several thousand kilometers south of the gyre. Typhoons were most common north and east of the gyre centers. More than 70% of gyres developed during large-amplitude MJO events, with a strong preference for Real-time Multivariate MJO ( RMM) phases 5-7. In boreal summer these phases contain circulation and convective anomalies that coincide most closely with those of the climatological monsoon trough. Gyres are most likely to form when an active, large-amplitude MJO event superposes with the monsoon trough in the presence of high sea surface temperature. Gyres exhibited 850hPa wind, height, and vorticity anomalies and surface latent heat flux anomalies that closely resembled the active Pacific-Japan pattern ( PJP). This was especially true during La Nina, even though no attempt was made to isolate the PJP. It is hypothesized that an active MJO modulates gyre formation, and the gyres project onto the active phase of the PJP as they move westward.